1. Field of the Invention
The present invention relates to an information reproducing apparatus such as a magneto-optic disc apparatus, in particular, to an information reproducing apparatus using PRML (Partial Response Maximum Likelihood) method and a reproducing method thereof.
2. Description of the Related Art
In a format of a record medium used for a recording/reproducing apparatus, sectors composed of an address portion and a data portion are defined. On a magneto-optic disc, the address portion is embossed. The data portion is magneto-optically recorded. The address portion is reproduced corresponding to the intensity of reflected light. The data portion is reproduced corresponding to magnetic Keer effect. In an information reproducing apparatus that does not use the Viterbi decoding method, since two types of signals that are theoretically different are reproduced by the same digitizing circuit, signals are recorded in such a manner that a signal reproduced from the address portion adequately matches a signal reproduced from the data portion.
In the reproducing system, the gain of an amplifier and of the characteristic of a filter for the address portion and those for the data portion are separately designated so that the signal reproduced from the data portion adequately matches the signal reproduced from the address portion and these signals are supplied to the digitizing circuit. However, in this method, the amplifier and the filter should be disposed for each of the address portion and the data portion. Thus, the circuit scale and the power consumption increase.
In recent years, in an information reproducing apparatus such as a magneto-optic disc apparatus, as a decoding method for decoding a signal reproduced from a record medium, Viterbi decoding method has been widely used. The Viterbi decoding method allows a bit error rate that takes place in decoding a reproduced signal including a white noise to decrease.
Next, an outline of the Viterbi decoding method will be described. A plurality of statuses corresponding to a recording method for a record medium are identified. The maximum likelihood status transition is selected at each read clock pulse by a calculating process corresponding to a signal reproduced from the record medium. Corresponding to the selected result, decoded data is generated as a sequence of decoded data values "1" or "0".
The calculating process corresponding to the reproduced signal is performed with reference to an amplitude reference value that depends on the type of Viterbi decoding method. When a reproduced signal is an ideal signal that is not affected by the fluctuation of the amplitude, the amplitude reference value logically depends on the type of Viterbi decoding method.
However, generally, due to various factors such as fluctuation of amplitude, an amplitude reference value for accurately decoding a reproduced signal fluctuates. However, since the fluctuation is not constant, the amplitude reference value cannot be shifted for improving the decoding accuracy. Thus, by updating the amplitude reference value corresponding to the fluctuation of the amplitude of the reproduced signal, the amplitude reference value should be adapted for the reproduced signal.
To do that, generally, the amplitude of a reproduced signal is detected by a means such as an envelope detector. Corresponding to the detected value, the amplitude reference value is updated at predetermined time intervals.
In a Viterbi decoder that adapts an amplitude reference value to a reproduced signal, even if the characteristic of a reproduced signal of the address portion is different from that of the data portion, amplitude reference values thereof are adapted to the respective reproduced signals.
An amplitude reference value is adapted for each sector as a data unit of the reproducing operation. As an initial value of an amplitude reference value adapted for a reproduced signal, a predetermined value such as a theoretical value is used. However, with such an initial value, when a new sector is reproduced, an amplitude reference value that has been adapted in the just preceding reproducing operation cannot be used.
Generally, the characteristic of a signal reproduced from the same record medium does not largely vary sector by sector. Thus, the adaptivity of the amplitude reference value adapted to a signal reproduced from a new sector by the just preceding reproducing operation is higher than the adaptivity of a predetermined value such as a theoretical value.
Thus, when a predetermined value such as a theoretical value is used as an initial value of an amplitude reference value, the amplitude reference value deviates from the characteristic of a reproduced signal in the convergence time period that depends on a correction coefficient used in the adapting process. In this period, the probability of which the accuracy of the Viterbi decoding process decreases is high.
As a result of the adapting process for an amplitude reference value to a reproduced signal, an abnormal amplitude reference value may take place. As one cause, the reproduced signal may largely disorder due to for example a defect of a record medium. When the adapting process is performed with such a disordered reproduced signal, an abnormal amplitude reference value may take place. With the abnormal amplitude reference value, since the accuracy of the Viterbi decoding process decreases, a signal may not be correctly reproduced.
Generally, when a signal is not correctly reproduced in the information reproducing apparatus, a sector that has not been correctly reproduced is reproduced once again (namely, a read-retry operation is performed). When the amplitude reference value at which the just preceding reproducing operation (namely, the reproducing operation for a sector that has not been correctly reproduced) has been performed is used as an amplitude reference value for the read-retry operation, as described above, the Viterbi decoding process is performed with the abnormal amplitude reference value. Thus, the probability of which a signal is not correctly reproduced is high.
In addition, the probability of which a sector that has not been correctly reproduced has a defect or the like is high. In the read-retry operation for such a sector, when the amplitude reference value is adapted, an abnormal amplitude reference value due to a defect or the like may take place. Thus, the probability of which a signal is not correctly reproduced is high.
On the other hand, when a plurality of sectors are reproduced, an address portion and a data portion are alternately reproduced. However, when amplitude reference values are used for the address portion and the data portion, just after the address portion has been reproduced, an amplitude reference value that has been adapted corresponding to the characteristic of a signal reproduced from the data portion is used. In contrast, just after the data portion has been reproduced, an amplitude reference value that has been adapted corresponding to the characteristic of a signal reproduced from the address portion is used.
Thus, just after the address portion or the data portion has been reproduced, the amplitude reference value may largely deviate from the characteristic of the reproduced signal. With the amplitude reference value, the decoding accuracy of the Viterbi decoding process may deteriorate.